Add option to write buffer directly to ctags input stream

[alerque]

See #568 for discussion where this was brought up, but this might be both more versatile and faster on as many platforms as support it. The current read file from disk or write the buffer to a cache file then read it both have significant speed trade-offs. The only trick with writing directly to STDIN might be dealing with Windows.

[wsdjeg]

@alerque in SpaceVim, we are using a forked version of tagbar, as I commend in https://github.com/SpaceVim/SpaceVim/issues/3683#issuecomment-694612528, we need a way to send context into ctags command STDIN instead of using temp file, many os do not allow user to read from temp file.

[alerque]

See #755 for another use case this would solve: tags for buffers which are either read from STDIN or are generated on the fly (e.g. man pages) and hence don't have file paths that we can pass to generate tags from.